// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// This file contains utility functions and classes that help the
// implementation, and management of the Callback objects.

#ifndef BASE_CALLBACK_INTERNAL_H_
#define BASE_CALLBACK_INTERNAL_H_

#include "base/atomic_ref_count.h"
#include "base/base_export.h"
#include "base/callback_forward.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"

namespace base {
namespace internal {
    template <CopyMode copy_mode>
    class CallbackBase;

    // BindStateBase is used to provide an opaque handle that the Callback
    // class can use to represent a function object with bound arguments.  It
    // behaves as an existential type that is used by a corresponding
    // DoInvoke function to perform the function execution.  This allows
    // us to shield the Callback class from the types of the bound argument via
    // "type erasure."
    // At the base level, the only task is to add reference counting data. Don't use
    // RefCountedThreadSafe since it requires the destructor to be a virtual method.
    // Creating a vtable for every BindState template instantiation results in a lot
    // of bloat. Its only task is to call the destructor which can be done with a
    // function pointer.
    class BASE_EXPORT BindStateBase {
    public:
        using InvokeFuncStorage = void (*)();

    protected:
        BindStateBase(InvokeFuncStorage polymorphic_invoke,
            void (*destructor)(const BindStateBase*));
        BindStateBase(InvokeFuncStorage polymorphic_invoke,
            void (*destructor)(const BindStateBase*),
            bool (*is_cancelled)(const BindStateBase*));
        ~BindStateBase() = default;

    private:
        friend class scoped_refptr<BindStateBase>;
        template <CopyMode copy_mode>
        friend class CallbackBase;

        bool IsCancelled() const
        {
            return is_cancelled_(this);
        }

        void AddRef() const;
        void Release() const;

        // In C++, it is safe to cast function pointers to function pointers of
        // another type. It is not okay to use void*. We create a InvokeFuncStorage
        // that that can store our function pointer, and then cast it back to
        // the original type on usage.
        InvokeFuncStorage polymorphic_invoke_;

        mutable AtomicRefCount ref_count_;

        // Pointer to a function that will properly destroy |this|.
        void (*destructor_)(const BindStateBase*);
        bool (*is_cancelled_)(const BindStateBase*);

        DISALLOW_COPY_AND_ASSIGN(BindStateBase);
    };

    // Holds the Callback methods that don't require specialization to reduce
    // template bloat.
    // CallbackBase<MoveOnly> is a direct base class of MoveOnly callbacks, and
    // CallbackBase<Copyable> uses CallbackBase<MoveOnly> for its implementation.
    template <>
    class BASE_EXPORT CallbackBase<CopyMode::MoveOnly> {
    public:
        CallbackBase(CallbackBase&& c);
        CallbackBase& operator=(CallbackBase&& c);

        explicit CallbackBase(const CallbackBase<CopyMode::Copyable>& c);
        CallbackBase& operator=(const CallbackBase<CopyMode::Copyable>& c);

        // Returns true if Callback is null (doesn't refer to anything).
        bool is_null() const { return bind_state_.get() == NULL; }
        explicit operator bool() const { return !is_null(); }

        // Returns true if the callback invocation will be nop due to an cancellation.
        // It's invalid to call this on uninitialized callback.
        bool IsCancelled() const;

        // Returns the Callback into an uninitialized state.
        void Reset();

    protected:
        using InvokeFuncStorage = BindStateBase::InvokeFuncStorage;

        // Returns true if this callback equals |other|. |other| may be null.
        bool EqualsInternal(const CallbackBase& other) const;

        // Allow initializing of |bind_state_| via the constructor to avoid default
        // initialization of the scoped_refptr.
        explicit CallbackBase(BindStateBase* bind_state);

        InvokeFuncStorage polymorphic_invoke() const
        {
            return bind_state_->polymorphic_invoke_;
        }

        // Force the destructor to be instantiated inside this translation unit so
        // that our subclasses will not get inlined versions.  Avoids more template
        // bloat.
        ~CallbackBase();

        scoped_refptr<BindStateBase> bind_state_;
    };

    // CallbackBase<Copyable> is a direct base class of Copyable Callbacks.
    template <>
    class BASE_EXPORT CallbackBase<CopyMode::Copyable>
        : public CallbackBase<CopyMode::MoveOnly> {
    public:
        CallbackBase(const CallbackBase& c);
        CallbackBase(CallbackBase&& c);
        CallbackBase& operator=(const CallbackBase& c);
        CallbackBase& operator=(CallbackBase&& c);

    protected:
        explicit CallbackBase(BindStateBase* bind_state)
            : CallbackBase<CopyMode::MoveOnly>(bind_state)
        {
        }
        ~CallbackBase() { }
    };

    extern template class CallbackBase<CopyMode::MoveOnly>;
    extern template class CallbackBase<CopyMode::Copyable>;

} // namespace internal
} // namespace base

#endif // BASE_CALLBACK_INTERNAL_H_
